Of the three known processes for the preparation of aromatic polycarbonates, that is to say the transesterification process, the two-phase boundary process and the process in a homogeneous phase system (also called the pyridine process), only the latter can be used directly if, in addition to diphenols, diols, that is to say compounds with two aliphatic OH groups, in particular higher-molecular diols, are also to be used to a very considerable extent for the preparation of the polycarbonates (see, for example, U.S. Pat. No. 3,161,615, U.S. Pat. No. 3,030,335, U.S. Pat. No. 3,287,442, Journal of Polymer Science, Part C, Polymer Symposia, No. 4 (1963), Part 1, pages 707-730, U.S. Pat. No. 3,641,200 and U.S. Pat. No. 3,843,708). In addition, the suspension process (see DT-OS (German Published Specification) No. 1,495,906 and U.S. Pat. No. 3,290,409), a modification of the process in a homogeneous phase system, can also be used directly.
In contrast, side reactions occur when the transesterification process is used, especially if polyester-diols are employed as the diols (see U.S. Pat. No. 3,161,615, column 5, lines 29-37, and U.S. Pat. No. 3,030,335, column 4, line 72-column 5, line 5).
The two-phase boundary process, in turn, requires prior conversion of the diols to be employed either into bis-chlorocarbonic acid esters (see U.S. Pat. No. 3,287,442, column 7, lines 32 et seq. and Journal of Polymer Science, Volumne 55 (1961), pages 343-352) or into diols with phenolic OH groups (see Journal of Polymer Science, Part C (1963), loc. cit. page 719).
The conversion last mentioned has hitherto been carried out successfully only as a two-stage process (see DT-OS (German Published Specification) No. 2,619,831 (LeA 16,933), DT-OS (German Published Specification) No. 2,636,784 (LeA 17,025), DT-OS (German Published Specification) No. 2,650,533 (LeA 17,516) and DT-OS (German Published Specification) No. 2,651,639 (LeA 17,535)).